Metal matrix composites for contacts on solar cells

ABSTRACT

A method for forming electrical contacts for a solar cell and a solar cell formed using the method is provided. The method includes forming a first metal layer over predefined portions of a surface of the solar cell; depositing a carbon nanotube layer over the first metal layer; and forming a second metal layer over the carbon nanotube layer, wherein the first metal layer, the carbon nanotube layer, and the second metal layer form a first metal matrix composite layer that provides electrical conductivity and mechanical support for the metal contacts.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 62/180,960, filed Jun. 17, 2015, the entirety of which isincorporated herein by reference.

GOVERNMENT RIGHTS

This disclosure was made with Government support under Contract No.FA9453-14-1-0242 awarded by the Air Force Research Laboratory. TheGovernment has certain rights in the invention.

TECHNICAL FIELD

Embodiments described herein relate generally to photonics, particularlyto generation, emission, control, and detection or sensing of light viaintegrated photonic source.

BACKGROUND

The advanced solar cells used in space vehicles today are rapidly movingtowards thin-film-based inverted metamorphic multijunction (IMM) solarcells mounted on flexible substrates. However, the IMM cells are moreprone to cracking than state-of-the-art triple junction cells. The cellcracking can lead to metal contact failure on IMM cells, compromisingthe power generation. This reliability issue is also becomingincreasingly more important for terrestrial thin-film-based,single-crystalline silicon (Si) solar cells. To illustrate, microcracksin crystalline-silicon-based photovoltaic cells have been characterizedusing electroluminescence after artificial aging and snow damage (seeFIG. 1). These microcracks can electrically disconnect areas of thecells and lead to substantial power loss (˜16%).

Thus, what is needed is an improved technique to mitigate the power lossand increase the lifetime of IMM cells.

SUMMARY

According to examples of the present disclosure, a method for formingelectrical contacts for a solar cell is provided. The method comprisesforming a first metal layer over predefined portions of a surface of thesolar cell; depositing a carbon nanotube layer over the first metallayer; and forming a second metal layer over the carbon nanotube layer,wherein the first metal layer, the carbon nanotube layer, and the secondmetal layer form a first metal matrix composite layer that provideselectrical conductivity and mechanical support for the solar cell.

According to examples of the present disclosure, a solar cell isprovided. The solar cell comprises a substrate and a metal matrixcomposite layer formed on a top surface of the substrate and configuredto provide electrical conductivity and mechanical reinforcement for themetal contacts, wherein the metal matrix composite layer comprises afirst metal layer formed on the top surface of the substrate, a carbonnanotube layer formed over the first metal layer, and a second metallayer formed over the carbon nanotube layer.

According to various examples, the first metal layer and the secondmetal layer comprise a metal selected from the group consisting of:silver, copper, and gold. The carbon nanotube layer is deposited byelectrodeposition, nanospreading, drop casting, or spray coating. Thecarbon nanotube layer is chemically functionalized with a carboxylicacid or an amine group prior to deposition to increase adhesion strengthto surrounding metal matrix and achieve efficient metal-nanotube stresstransfer. The solar cell can be a standard multijunction solar cell, aninverted metamorphic multijunction solar cell, or a silicon-basedterrestrial solar cell. The method further comprising forming a secondmetal matrix composite layer over the first metal matrix compositelayer. The solar cell comprises a III-V compound semiconductor substrateor a silicon substrate. The forming the first metal layer is performedwith a cathodic current density of about ±2-mA/cm² to about ±10-mA/cm²and with silver ion concentration, pH, and bath temperature heldconstant. The silver ion concentration can be about 1.5 to about 2.3oz/gal, the operating pH can be about 9.1, and the temperature can beabout 25° C. The carbon nanotube layer comprises a plurality ofmultiwalled carbon nanotubes, single-walled nanotubes, or a mixture ofboth multiwalled and single-walled nanotubes. The method furthercomprising forming a silver seeding layer by physical vapor depositionand subsequent electroplating of silver to produce a mirror-like finish.The carbon nanotube layer is chemically functionalized with thecarboxylic acid using an acid treatment in a 1:3 HNO₃:H₂SO₄ mixture toproduce a negative surface charge. The amine group comprises NH₂ toproduce a positive surface charge by sonicating the carbon nanotubelayer in a 2.8 M NH₄Cl aqueous solution. The first metal layer that isformed is about 3-μm in thickness on the solar cell substrate andwherein the carbon nanotube layer that is chemically functionalized isdeposited at about −0.5 mA/cm² for about 15 minutes. The first metallayer and the second metal layer comprise silver and the nanospreadingcomprises wherein the forming the first metal layer compriseselectroplating a 3 μm thick silver layer on a 100 nm thick physicalvapor deposition silver seeding layer on a solar cell substrate; whereindepositing the carbon nanotube layer comprises depositing the carbonnanotube layer that is chemically functionalized over the first metallayer by dragging a meniscus of a microliter suspension droplets ofcarbon nanotubes trapped between the GaAs substrate and moving blade ata constant velocity. The method further comprises repeating thedeposition of the carbon nanotube layer that is chemicallyfunctionalized about five times at a blade pull speed of about 1 toabout 100 μm/s. The method further comprising forming another 3-μm thicklayer of silver over carbon nanotube layers to produce a totalmetal-carbon-metal structure of 6-μm thickness. The carbon nanotubelayer forms a network and the second metal layer is at least partiallyin the network. In the method where depositing a carbon nanotube layer,the method comprises forming a network of carbon nanotubes. In themethod where forming a second metal layer over the carbon nanotubelayer, the method comprises at least partially embedding the secondmetal layer within the network.

Additional advantages of the embodiments will be set forth in part inthe description which follows, and in part will be understood from thedescription, or may be learned by practice of the embodiments. Theadvantages will be realized and attained by means of the elements andcombinations particularly pointed out in the appended claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the embodiments, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the presentteachings and together with the description, serve to explain theprinciples of the disclosure.

FIG. 1 shows an electroluminescence image of a micro-crackedconventional PV module, where the dark regions are electrically inactiveareas.

FIG. 2 shows a representation of a solar cell with a crack through thelayers of the cell and across the metal gridlines.

FIGS. 3A-3G shows a method of forming a metal matrix composite structureon a solar cell substrate, according to examples of the presentdisclosure.

FIGS. 4A and 4B shows a solar cell substrate with a Ag seed layer formedthereon and Ag-CNT-Ag MMC gridlines formed thereon, respectively,according to examples of the present disclosure.

FIG. 5A is a plot of cathode potential (V_(c)) vs. cathode currentdensity (J_(c)), where the region between Point D and Point L definesthe ideal operating window where the finish of plated Ag is bright,according to examples of the present disclosure.

FIG. 5B is an image of a PVD Ag seeding layer and subsequentelectroplating of Ag that results in a mirror finish, according toexamples of the present disclosure.

FIG. 6 is a plot of cyclic voltammetry measurements on the as received,amine-terminated, and carboxylated CNTs with corresponding images of theCNTs solutions, according to examples of the present disclosure.

FIG. 7A is a SEM image of COOH-terminated CNTs deposited on 40-nm-thick,sputter-coated Ag to clearly show Ag dissolution during negativelycharged CNT deposition on positively biased working electrode, accordingto examples of the present disclosure.

FIG. 7B is a I-V characteristic curve, where the shaded region indicatesthe optimum operating range above which V_(c) rapidly rises withincreasing J_(c), according to examples of the present disclosure.

FIGS. 8A and 8B are SEM images of COOH-terminated CNTs deposited onelectroplated Ag, using the nanospreader technique, according toexamples of the present disclosure.

FIGS. 8C and 8D are cross-sectional SEM images of Ag-CNT-Ag compositestructure, where the CNTs, indicated by arrows, exhibit good adhesion tothe surrounding Ag matrix, according to examples of the presentdisclosure.

FIG. 9 is a plot of the linear relation between the pull speed andcorresponding surface coverage of CNTs using the nanospreader method,according to examples of the present disclosure.

FIG. 10A is a plot of CNTs loading fraction vs. surface coverage withstandard deviation bars, according to examples of the presentdisclosure.

FIG. 10B is histogram of gray values used to computer the plot of FIG.10A.

FIG. 11A is a log plot of resistance vs. gap across fractured MMCgridlines with three different CNT surface coverages, according toexamples of the present disclosure.

FIG. 11B shows two printed circuit boards used in the test from whichFIG. 11A was generated.

FIG. 12A is a plot of results of a strain failure test of a MMC gridline(96% CNT surface coverage using drop cast method), according to examplesof the present disclosure.

FIG. 12B shows two printed circuit boards used in the test from whichFIG. 12A was generated.

FIG. 13 is a plot of maximum and average gap widths achieved before lostconnection as a function of CNT surface coverage and loading fraction,according to examples of the present disclosure.

FIG. 14 are SEM images of CNTs anchored in Ag layers showing CNTsbridging cracks up to 9-μm-wide, according to examples of the presentdisclosure.

FIG. 15A is a plot of average gap width versus pull cycle, according toexamples of the present disclosure.

FIG. 15B shows a plot of measured current versus applied voltage,according to examples of the present disclosure.

FIG. 16 are another set of SEM images of CNTs anchored in Ag layersshowing CNTs bridging cracks up to 9-μm-wide, according to examples ofthe present disclosure.

FIGS. 17A and 17B are plots of current density versus voltagecharacteristics of commercial triple junction solar cells before andafter cracks form, respectively, according to examples of the presentdisclosure.

FIG. 18 is a table of performance values for control samples and samplesaccording to examples of the present disclosure (percentage deviationfrom the control samples).

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present embodiments,examples of which are illustrated in the accompanying drawings. Whereverpossible, the same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the embodiments are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements. Moreover, all ranges disclosed hereinare to be understood to encompass any and all sub-ranges subsumedtherein. For example, a range of “less than 10” can include any and allsub-ranges between (and including) the minimum value of zero and themaximum value of 10, that is, any and all sub-ranges having a minimumvalue of equal to or greater than zero and a maximum value of equal toor less than 10, e.g., 1 to 5. In certain cases, the numerical values asstated for the parameter can take on negative values. In this case, theexample value of range stated as “less than 10” can assume negativevalues, e.g., −1, −2, −3, −10, −20, −30, etc.

The following embodiments are described for illustrative purposes onlywith reference to the figures. Those of skill in the art will appreciatethat the following description is exemplary in nature, and that variousmodifications to the parameters set forth herein could be made withoutdeparting from the scope of the present embodiments. It is intended thatthe specification and examples be considered as examples only. Thevarious embodiments are not necessarily mutually exclusive, as someembodiments can be combined with one or more other embodiments to formnew embodiments. It will be understood that the structures depicted inthe figures may include additional features not depicted for simplicity,while depicted structures may be removed or modified.

Generally speaking, examples of the present disclosure provide for asolar cell having metal-carbon-nanotube metal matrix composites formetal contacts for metal gridlines and methods of making thereof. Themetal films imbedded with multiwalled carbon nanotubes (CNTs) are knownas metal matrix composites (MMCs), and can provide for reinforcedmechanical strength against stress-induced cracks in the solar cells andgapping capability to electrically bridge microcracks that form due tothermal and mechanical stress. The metal can include, but is not limitedto, silver, gold, and copper. Other metals that have good electricalconductivity can also be used. The high mechanical strength and goodelectrical/thermal conductivities of CNTs make them a suitable componentin the MMC to reinforce the metal gridlines. However, in order toharness these properties, CNTs have to be properly imbedded within themetal matrix.

FIG. 1 shows an electroluminescence image of a micro-crackedphotovoltaic (“PV”) module, where the dark regions are electricallyinactive area. FIG. 2 shows a representation of a multi junction solarcell with a crack running through the layers of the solar cell, as wellas across the metal gridlines.

FIGS. 3A-3G show a method of forming a MMC microstructure, according toexamples of the present disclosure. In this example, the MMCmicrostructure is an Ag-CNT-Ag layer-by-layer (LBL) structure, in whichfunctionalized CNTs are deposited on electroplated Ag in an alternatingfashion. The substrate on which the MMC microstructure is formed can bea III-V compound semiconductor substrate or a silicon substrate,including, but are not limited to, Si, Ge, GaAs, or InGaP. The CNT layercan comprise a plurality of multiwalled carbon nanotubes, single-wallednanotubes, or a mixture of both multiwalled and single-walled nanotubes.While the below examples show silver as the metal for the metal layers,the metal can be copper, gold, etc., as discussed above. As shown inFIG. 3A, a substrate 305 of a solar cell is subjected to anelectrochemical Ag deposition process, using a plating solution 302,such as a commercially available plating solution, E-brite 50/50 RTP,using controlled plating conditions, through the manipulation ofparameters including pH, temperature, ionic strength, and currentdensity. The silver ion concentration can be about 1.5 to about 2.3oz/gal, the operating pH can be about 9.1, and the temperature can beabout 25° C. Other methods of depositing the metal can be used,including, but are not limited to, sputter deposition or screenprinting. The inventors performed experiments to optimize the platingconditions and found that the main variable is cathodic current density(J_(c)), while Ag ion concentration, pH, and bath temperature are heldconstant. FIG. 5A shows the cathode potential (V_(c)) vs. currentdensity (J_(c)) with corresponding images of Ag electroplated on GaAssubstrates. The region between D and L define the ideal operating windowwhere the plated Ag appears bright, which is desirable. A currentdensity of about 2 mA/cm² to about 10 mA/cm²±0.5 mA/cm², and moreparticularly, of about 3 mA/cm²±0.5 mA/cm² results in a bright finish.FIG. 3B shows the silver layer 310 formed on a top surface of substrate305 resulting in a silver plated substrate after the electroplatingprocess. The silver layer can have a thickness of about 3 μm±0.5 μm.

In some examples, an initial seeding layer can be formed on a topsurface of the substrate, which can beneficially affect the final filmquality. FIG. 4A shows a solar cell substrate with an Ag seed layer 405,according to examples of the present disclosure. Various thin (˜100 nm)seeding layers can be used, including, but are not limed to,sputter-coated Ti, sputter-coated Ag, and Ag deposited by physical vapordeposition (PVD). The PVD Ag seeding layer and subsequent electroplatingof Ag result in a mirror finish, as shown in FIG. 5B.

In some examples, CNTs can be incorporated into the metal matrix throughchemical functionalization, which allows for chemical bonding betweenCNTs and their surrounding environment. The chemical functionalizationenhances the wettability and adhesion of CNTs to metal. For example, thefunctionalized CNTs can be suspended in water without any additives, andthe aqueous solution can be spray-coated directly onto the substrates.In one example, CNTs are functionalized with a carboxylic acid, i.e.,COOH, for negative surface charge, following a standard acid refluxmethod. The maximum electrokinetic mobility of CNTs is achieved throughan acid treatment in a 1:3 HNO₃:H₂SO₄ mixture. In another example, CNTsare functionalized with an amine group, i.e., NH₂, for positive surfacecharge by sonicating CNTs in 2.8 M NH₄Cl aqueous solution. Carboxylationof CNTs produces stable, homogenous aqueous solutions of CNT. FIG. 6 isthe cyclic voltammetry measurements on the as-received substrateswithout metal gridlines formed thereon 605, NH₂-functionalized 610, andCOOH-functionalized CNTs 615 with corresponding images of the CNTssolutions. It was observed that the highest overall integrated chargeassociated with the carboxylated CNTs 615, which indicates thatcarboxylated CNTs assume the highest amount of surface charge andpresumably the highest electrokinetic mobility compared to neutral andamine-terminated CNTs.

FIG. 3C shows the deposition of carbon nanotubes producing a carbonnanotube layer 315 over the silver layer 310 of substrate 305. Thecarbon nanotube layer 315 can be deposited using the various methodsdescribed below, including spray coating. FIG. 3D shows carbon nanotubelayer 315, silver layer 310, substrate 305 after nanotube deposition.FIG. 3E shows a top view of FIG. 3D. FIG. 3F shows the formation of asecond silver layer using the process of the first silver layer. FIG. 3Gshows the completed solar cell with first silver layer 310, carbonnanotube layer 315, and second silver layer 320 forming metal matrixcomposite 325. The carbon nanotubes form a network and the second metallayer, i.e., silver layer 320, is at least partially embedded in thenetwork. The process can be repeated a number of times forming more thanone metal matrix composite layer. FIG. 4B shows the solar cell of FIG.4A with Ag-CNT-Ag MMC gridlines 410 formed over the Ag seed layer 405,according to examples of the present disclosure.

For the microstructural comparison, COOH-terminated CNTs were used tocreate a LBL Ag composite microstructure. First, about 1-μm to about4-μm, and more particularly about 2-μm±0.5-μm thick Ag film iselectroplated on GaAs followed by electrochemical deposition of CNT-COOHat about −0.1 mA/cm² to about −10 mA/cm², and more particularly about−0.5 mA/cm² for about 15 min, where the negative sign for the currentdensity is to indicate that the working electrode is being biased with apositive potential to draw in negatively charged carboxylated carbonnanotubes. That is, during metal deposition, positively charged metalions are drawn towards the negatively biased working electrode, whereasduring CNT-COOH (carboxylated carbon nanotubes) deposition, negativelycharged CNTs are drawn towards a positively biased electrode. The sampleis then electroplated with another 2-μm-thick Ag layer. Due to thenegative surface charge on CNT-COOH, a positive bias must be applied tothe working electrode (i.e., anode) to deposit CNTs. Consequently, theplated Ag dissolves back into the solution during CNT deposition. FIG.7A is a SEM image of COOH-terminated CNTs 705 deposited on about 10-nmto about 100-nm, and more particularly about 40-nm±10-nm-thick,sputter-coated Ag 710 over a GaAs substrate 715 to show Ag dissolutionduring negatively charged CNT deposition on positively biased workingelectrode. FIG. 7B is a I-V characteristic curve, where the shadedregion 720 indicates the optimum operating range above which V_(c)rapidly rises with increasing J_(c).

As an alternate method to electrochemical deposition, functionalizedCNTs can be deposited using a nanospreader technique. A 2-μm-thick Aglayer is first electroplated on a 100-nm-thick PVD Ag seeding layer onGaAs samples. The nanospreader technique is then used to deposit a thinlayer of CNTs by dragging at a constant velocity the meniscus ofmicroliter suspension droplets of CNTs trapped between the substrate anda moving Teflon blade. One to about 100 layers, and more particularlyfive layers of carboxylated CNTs are successively deposited at a bladepull speed of 1 to about 100-μm/s, and more particularly about10-μm/s±3-μm/s. The samples are then plated with another 2-μm-thick Aglayer, creating a MMC film with a total thickness of ˜4 μm. FIGS. 8A and8B are SEM images of COOH-terminated CNTs deposited on electroplated Ag,using the nanospreader technique. FIGS. 8C and 8D are cross-sectionalSEM images of Ag-CNT-Ag composite structure. The CNTs indicated byarrows exhibit good adhesion to the surrounding Ag matrix. FIGS. 8A-8Dshow the nanospreader sample before the 2^(nd) Ag layer is deposited(FIGS. 8A and 8B) and a cross sectional view after the 2^(nd) Ag layeris deposited (FIGS. 8C and 8D). The cross-sectional SEM images show CNTsintercalating the Ag matrix. These images suggest that surfacefunctionalized CNTs adhere well to Ag and that these CNTs would bridgethe microcracks that form upon strain-induced failure.

The inventors investigated a range of deposition rates using thenanospreader technique with pull speeds ranging from 2 to 30 μm/s anddroplet volumes ranging from 10 to 50 μl. Through the manipulation ofpull speed, CNT surface coverage ranging from 12% to 86% was obtained.FIG. 9 shows a plot of the linear relation between the pull speed andcorresponding surface coverage of CNTs using the nanospreader methodfrom the experiments conducted. The surface coverage is defined as thepercentage of substrate surface covered by CNTs, ignoring any CNToverlap. The corresponding surface coverage is quantified though digitalimage processing of SEM images. Using ImageJ program, the SEM images areconverted into a matrix of gray scale values with the correspondingxy-coordinates. The gray scale values are then plotted in a histogram,as shown in FIG. 10B and a cutoff gray value is assigned per image. Thiscutoff value represents the gray scale value of an open space; hence allgray values less than this cutoff value are counted and divided by thetotal counts, giving the percentage surface coverage. It is noted thatthis cutoff value is not a constant due to the variability in contrastand brightness settings from image to image.

While the nanospreader technique offers precise control over the CNTsurface coverage, the slow throughput may limit its manufacturability.As an alternative method of deposition, a drop casting method using asolution of carboxylated CNTs was investigated using a range ofdifferent CNT loading fractions, i.e., CNT solution concentration, anddigitally analyze the subsequent surface coverage, as shown in FIG. 10B.Digital analysis was performed using ImageJ program. FIG. 10A shows howthe CNT loading fraction translates to the percentage surface coverage,and in particular shows CNTs loading fraction versus surface coveragewith standard deviation bars. As shown in FIG. 10A, past point H (>9g/cm²), the surface coverage plateaus, asymptotically approaching 100%.A different CNT loading fraction was used to prepare MMC gridlines (each1-mm-wide) to create the LBL microstructure. First, a 2-μm-thick Ag filmis electroplated on a 100-nm-thick PVD Ag seeding layer on an InPsubstrate. The solution of CNT-COOH is then drop-cast at roomtemperature. Another 2-μm-thick Ag layer is finally electroplated ontop.

A strain failure test was performed to determine how well the MMCsamples can maintain the electrical conductivity upon mechanicalfracture. A set of four parallel MMC gridlines, deposited on an InPsubstrate, were first mounted on two printed circuit boards using anadhesive (FIGS. 11B and 12B). Upon curing the adhesive, the substratewas scribed with a diamond tip to generate a crack that propagatesacross the substrate backside, orthogonal in direction to the MMCgridlines. The cracked substrate was then attached to a linear stagecontrolled by a stepper motor. The resistance across each of the MMCgridlines was continuously recorded as the gridlines are pulled apart atmicron increments until the electrical resistivity approaches infinityupon plastic failure. Following the first electrical disconnect, the gapwas incrementally closed in reverse until the electrical connection isreestablished across the gridlines; the substrate was then pulled apartagain. This process was repeated until no further change is seen in thegap width at which the connection is lost. FIG. 11A shows a log plot ofresistance versus gap across fractured MMC gridlines with threedifferent CNT surface coverages. The connection was reestablished at 49μm, 9 μm, and 5 μm for samples with CNT surface coverage of 72%, 55%,and 35%, respectively. FIG. 12A shows a strain failure test results of aMMC gridline pull test with 96% CNT coverage using the drop cost method.After the second pull test, the maximum gap, at which the electricalconnection is lost, is reduced by ˜34% but subsequently remainedconstant with additional pull tests.

Using the drop casting method, a set of MMC samples were prepared withdifferent CNT surface coverage. The MMC gridlines were assessed usingthe strain failure test. Four gridlines with the same CNT loading weretested and the maximum and average gaps before the loss of electricalconnection were recorded in FIG. 13, which shows a plot of gap width inμm versus surface coverage percentage. The pull tests reveal that theelectrical connection is maintained across larger gaps with higher CNTsurface coverage, reaching a maximum of 42-μm-wide gap. As the CNTsurface coverage increases from 35% to 96%, a monotonic increase in theaverage gap was observed. Electroplated Ag lines without CNTs were alsoanalyzed as control, using the strain failure test. Unlike MMC films,the Ag gridlines did not withstand the initial crack generated on thesubstrate (˜4-μm-wide), and the electrical connection is immediately andirrecoverably lost.

It is evident that the incorporation of CNTs within the Ag matrixenhances the electrical conductivity of metal lines upon fracture. Tovisualize this capability, a MMC film was intentionally fractured andthe cracks were examined under SEM. It was observed that CNTs of variouslengths bridging gaps ranging from 0.2 to 9 μm. FIG. 14 shows a seriesof SEM images of the CNTs anchored in Ag layers that show CNTs bridgingcracks up to 9 μm wide. The CNTs are anchored in the Ag matrix,indicating a good adhesion between the functionalized CNTs and the Agmatrix.

Mechanical characterization was performed on the MMC films using aBerkovich round tip nanoindenter. Stress-strain curves were obtained adepth profile of 2 μm. 4-μm-thick MMC samples were prepared usingnanospreader and drop casting methods to deposit CNTs at various loadingfractions. The nanoindentation analysis revealed that the composite filmhas a lower elastic modulus (10 GPa) than pure silver (73 GPa), which iscontrary to initial prediction given the high elastic modulus of CNTs(1000 GPa). The lower elastic modulus is attributed to theelectroplating process of silver, in which hydrogen is incorporated andtrapped within the composite. Finite element analysis also corroboratesthis speculation, where the elastic modulus near 10 GPa is predictedwith approximately 4% void fraction. While the composite elastic modulusis lower than that of pure silver, the strain failure tests show thatcarbon nanotubes can bridge 10 to 42-μm-wide microcracks, maintainingthe electrical conductivity.

FIG. 15A shows a plot of average gap width versus pull cycle. FIG. 15Bshows a plot of measure current versus applied voltage. The plot of FIG.15A shows how the carbon nanotubes can bridge the crack in the metallayers, and how this bridging process can be repeated as the brokenmetal lines are pulled close together and pulled apart in a repeatcycle.

FIG. 16 shows another series of SEM images showing the CNTs bridginggaps in the solar cell, and in particular shows a cross-sectional viewsof a typical metal matrix composite stack integrated onto a commercialtriple junction cell.

FIG. 17A and 17B shows plots of current density versus voltage fordifferent test samples under different stress conditions, and inparticular, shows how the metal matrix composite lines according to thepresent examples help maintain the solar cell performance withoutdegradation even after substrate fracture, whereas the standardconventional metal lines used in commercial products lead to immediatedegradation in solar cell performance after substrate fracture.

FIG. 18 shows a table that compares the commercial metal line,electrodeposited metal line, and metal matrix composite lines, where theMMC shows the least change (represented here as % delta) after substratefracture. That is, there is no noticeable degradation in solar cellperformance when the composite metal matrix is used in place ofcommercial or electroplated metal lines.

In summary, metal-CNT-metal matrix composite (MMC) films are providedthat maintain electrical conductivity upon mechanical fracture of thesubstrate. The composite lines are capable of bridging cracks in theunderlying semiconductor substrates up to 42 μm. That is, theincorporation of CNTs within the metal matrix renders the metal linesmore resilient against mechanical failures, compared to the 100% Aglines. In addition, to maintaining the electrical conductivity over thestress-induced cracks, the composite films can reestablish theelectrical connection when the cracks close up. This “self-healing”behavior of MMC gridlines is a strongly desirable characteristic inconsideration of the extreme temperature fluctuations encountered inspace operations, in which the PV cells undergo constant expansion andcontraction.

While the embodiments have been illustrated respect to one or moreimplementations, alterations and/or modifications can be made to theillustrated examples without departing from the spirit and scope of theappended claims. In addition, while a particular feature of theembodiments may have been disclosed with respect to only one of severalimplementations, such feature may be combined with one or more otherfeatures of the other implementations as may be desired and advantageousfor any given or particular function.

Furthermore, to the extent that the terms “including”, “includes”,“having”, “has”, “with”, or variants thereof are used in either thedetailed description and the claims, such terms are intended to beinclusive in a manner similar to the term “comprising.” As used herein,the phrase “one or more of”, for example, A, B, and C means any of thefollowing: either A, B, or C alone; or combinations of two, such as Aand B, B and C, and A and C; or combinations of three A, B and C.

Other embodiments will be apparent to those skilled in the art fromconsideration of the specification and practice of the descriptionsdisclosed herein. It is intended that the specification and examples beconsidered as exemplary only, with a true scope and spirit of theembodiments being indicated by the following claims.

What is claimed is:
 1. A method for forming electrical contacts for asolar cell, the method comprising: forming a first metal layer overpredefined portions of a surface of the solar cell; depositing a carbonnanotube layer over the first metal layer; and forming a second metallayer over the carbon nanotube layer, wherein the first metal layer, thecarbon nanotube layer, and the second metal layer form a first metalmatrix composite layer that provides electrical conductivity andmechanical support for the metal contacts.
 2. The method of claim 1,wherein the first metal layer and the second metal layer comprise ametal selected from the group consisting of: silver, copper, and gold.3. The method of claim 1, wherein the carbon nanotube layer is depositedby electrodeposition, nanospreading, drop casting, or spray coating. 4.The method of claim 3, wherein the carbon nanotube layer is chemicallyfunctionalized with a carboxylic acid or an amine group prior todeposition to increase adhesion strength to surrounding metal matrix andachieve efficient metal-nanotube stress transfer.
 5. The method of claim1, wherein the solar cell is an inverted metamorphic multijunction solarcell.
 6. The method of claim 1, further comprising forming a secondmetal matrix composite layer over the first metal matrix compositelayer.
 7. The method of claim 3, wherein the solar cell comprises aIII-V compound semiconductor substrate.
 8. The method of claim 1,wherein the forming the first metal layer is performed with a cathodiccurrent density of about 2-mA/cm² to about 10-mA/cm² and with silver ionconcentration, pH, and bath temperature held constant.
 9. The method ofclaim 1, wherein the carbon nanotube layer comprises a plurality ofmultiwalled carbon nanotubes, single-walled nanotubes, or a mixture ofboth multiwalled and single-walled nanotubes.
 10. The method of claim 1,further comprising forming a silver seeding layer by physical vapordeposition and subsequent electroplating of silver to produce amirror-like finish.
 11. The method of claim 4, wherein the carbonnanotube layer is chemically functionalized with the carboxylic acidusing an acid treatment in a 1:3 HNO₃:H₂SO₄ mixture to produce anegative surface charge.
 12. The method of claim 4, wherein the aminegroup comprises NH2 to produce a positive surface charge by sonicatingthe carbon nanotube layer in a 2.8 M NH₄Cl aqueous solution.
 13. Themethod of claim 7, wherein the first metal layer that is formed is about3-μm in thickness on the solar cell substrate and wherein the carbonnanotube layer that is chemically functionalized is deposited at about−0.5 mA/cm² for about 15 minutes.
 14. The method of claim 3, wherein thefirst metal layer and the second metal layer comprise silver and thenanospreading comprises: wherein the forming the first metal layercomprises electroplating a 2 μm thick silver layer on a 100 nm thickphysical vapor deposition silver seeding layer on a solar cellsubstrate; wherein depositing the carbon nanotube layer comprisesdepositing the carbon nanotube layer that is chemically functionalizedover the first metal layer by dragging a meniscus of a microlitersuspension droplets of carbon nanotubes trapped between the first metallayer and moving blade at a constant velocity.
 15. The method of claim14, further comprising repeating the deposition of the carbon nanotubelayer that is chemically functionalized about five times at a blade pullspeed of about 1 to about 100 μm/s.
 16. The method of claim 14, furthercomprising forming another 3-μm thick layer of silver over carbonnanotube layers to produce a total metal-carbon-metal structure of 6-μmthickness.
 17. The method of claim 1, where depositing a carbon nanotubelayer comprises forming a network of carbon nanotubes.
 18. The method ofclaim 17, where forming a second metal layer over the carbon nanotubelayer comprises at least partially embedding the second metal layerwithin the network.
 19. A solar cell comprising: a substrate; and ametal matrix composite layer formed on a top surface of the substrateand configured to provide electrical conductivity and mechanicalreinforcement for the metal contacts, wherein the metal matrix compositelayer comprises a first metal layer formed on the top surface of thesubstrate, a carbon nanotube layer formed over the first metal layer,and a second metal layer formed over the carbon nanotube layer.
 20. Thesolar cell of claim 17, wherein the carbon nanotube layer is chemicallyfunctionalized with a carboxylic acid or an amine group prior todeposition to increase adhesion strength to metal contacts and achieveefficient metal-nanotube stress transfer.